Thermoplastic polyesters are essentially linear polymeric molecules containing in-chain ester groups, and are known to be truly versatile materials, being commonly used as fibers, plastics and films; in composites and elastomers; and as coatings. The production of polyesters by condensation of polyfunctional carboxylic acids with polyfunctional alcohols (or their ester-forming derivatives) is well known in the art, and is e.g. described in Encyclopaedia of Polymer Science and Engineering, 2nd ed, volume 12, John Wiley and Sons, New York (1988).
The most commonly used polyester is polyethylene terephthalate (PET). The first PET production in 1940's employed dimethyl terephthalate (DMT) and monoethylene glycol (EG) as precursors. However, most production plants currently use pure grade terephthalic acid (PTA) and monoethylene glycol (EG) as raw materials, because of process economic reasons. In this case, first a low molecular weight prepolymer is formed by esterification of PTA with EG to form diethyleneglycol terephthalate and oligomers (DGT), with water as the main by-product being distilled off (step 1). This step is generally self-catalysed, but may be accelerated by adding catalyst. DGT is further subjected to polycondensation by transesterification and esterification reactions to form higher molecular weight polyester (step 2). Towards the end of polycondensation, transesterfication reactions become dominant. In this step, DGT is heated to about 280° C. under high vacuum to carry out the melt-phase polycondensation reaction with removal of the reaction byproducts, namely EG and water. Because transesterification is a slow reaction, the polycondensation step is generally catalysed. This catalyst can be added in step 2, but it can also already be included in step 1. The melt is discharged and chipped after it has reached desired molecular weight values, reflected by intrinsic viscosity (IV) values.
Industrial-scale PET production is generally based on a continuous PTA system employing several reactors in series, as described for example by S. M. Aharoni in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 2, Editor S. Fakirov, Wiley-VCH, 2002; and by V. B. Gupta and Z. Bashir in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 7, Editor S. Fakirov, Wiley-VCH, 2002. This system uses a vessel in which EG, PTA, catalyst and additives are mixed; one or two esterification reactors; one or two pre-polycondensation reactors, followed by a high-vacuum finisher reactor for the final stages of polycondensation. The polyester formed may be extruded into filaments, quenched under water and cut to form amorphous chips.
PET is mainly used in industry for production of textile fibers, filaments, films and bottle grade chips. PET used in film and fiber applications typically has an IV in the range of 0.58 to 0.64 dL/g; PET films and fibers can be produced directly by extruding the melt from the poycondensation reactor. For PET bottle grade resin, polymers with IV in the range of 0.75 to 0.85 dL/g, and with low residual acetaldehyde are generally required. In this case, a split process is used to attain this IV value while minimising the amount of acetaldehyde. The general practice is to make polymer chips with an intermediate IV of about 0.63 dL/g by melt polycondensation, and then increase the IV to about 0.75 to 0.85 dL/g by subsequent solid-state polycondensation (SSP). This split procedure allows production of a high IV resin with minimal quantities of acetaldehyde, which is a degradation by-product that may affect the taste of beverages packed in PET bottles. Diethylene glycol (DEG) is a diol generated from ethylene glycol via a side reaction and is also incorporated in the PET chain. Presence of DEG as comonomer reduces the glass transition and melting temperature of the PET, and too high levels are undesirable. The melt phase and SSP technology is described for example by V. B. Gupta and Z. Bashir, in “Handbook of Thermoplastic Polyesters”, vol. 1, chapter 7, Editor S. Fakirov, Wiley-VCH, 2002.
The catalysts currently employed in industrial PET production are catalysts generally based on antimony (Sb), mostly antimony triacetate or antimony trioxide. A disadvantage of using antimony-based catalyst compounds is the greyish color of PET that results from partial precipitation of antimony metal. Moreover, antimony is rather expensive and it shows some environmental concerns. Also, the usage of germanium dioxide as catalyst is limited due to its high cost because of scanty reserves, though this catalyst gives polyesters with good clarity. Various titanium (Ti) based compounds have also been proposed as polycondensation catalysts, because they are relatively inexpensive and safe. Described titanium-based catalysts include tetra-n-propyl titanate, tetraisopropyl titanate, tetra-n-butyl titanate, tetraphenyl titanate, tetracyclohexyl titanate, tetrabenzyl titanate, tetra-n-butyl titanate tetramer, titanium acetate, titanium glycolates, titanium oxalates, sodium or potassium titanates, titanium halides, titanate hexafluorides of potassium, manganese and ammonium, titanium acetylacetate, titanium alkoxides, titanate phosphites etc.
Documents EP0699700, U.S. Pat. No. 3,962,189 and JP52062398 describe production of polyesters using Sb-free Ti-based catalyst compositions, optionally containing Mn—, Co—, P— and/or other compounds. The colour of the obtained polyester generally shows a yellowish colour, and relatively high amounts of acetaldehyde and cyclic by-products are formed.
U.S. Pat. No. 6,372,879 describes polyester polycondensation in the presence of a Ti-based catalyst and a catalyst enhancer. The composition comprises titanyl oxalate catalyst consisting of a combination of lithium or potassium titanyl oxalates with Sb-trioxide, -triacetate or -triglycoxide; a metallic oxalate catalyst enhancer; and optionally, Sb-based co-catalyst.
U.S. Pat. No. 6,143,837 discloses a process for preparation of polyester resins using Ti alkoxides, acetylacetonates, dioxides and phosphites. Though the Ti-catalyst is about four times more active than Sb-catalyst, in order to reduce yellow coloration in the polyester cobalt compounds and other organic colorants were added. Also dianhydride of an aromatic tetra carboxylic acid was added to compensate for the low reactivity of the Ti-based catalyst towards solid-state polycondensation (SSP); a disadvantage being that this addition leads to branching.
Titanium glycolates are also described as a catalyst for PET in R. Gutman, Textile Praxis International 1, 1989, 29-33, and EP1585779. Disadvantages include tendency for yellowing, and reduced activity in SSP.
U.S. Pat. No. 6,034,203 discloses a process to produce polyesters in the presence of a catalyst having the formula MxTi(III)Ti(IV)yO(x+3+4y)/2, wherein M is an alkali metal, Ti(III) is Ti in +3 oxidation state, Ti(IV) is Ti in +4 oxidation state, x and y≧0, and when x=0, then y<½. Ti2O3 and LixTiO2 were employed in the examples as catalysts; PET obtained with these catalysts shows yellowing.
WO95/18839 discloses a production process for polyesters wherein specific TiO2/SiO2 or TiO2/ZrO2 co-precipitates are applied as catalyst, showing higher activity then Sb-based systems.
EP0736560 discloses also titanate catalysts of formula (MenO)x.(TiO2)y.(H2O)z, wherein Me is an alkaline earth or alkali metal, and having specific particle size for production of polyesters.
JP2000119383 discloses a process to manufacture polyesters by using a nano-sized titanium dioxide as polycondensation catalyst, with particle size of 100 nm or less and a specific surface area of 10 m2/g or more. The activity of this catalyst is comparable with the activity of an antimony catalyst; however aggregation of nano crystals is generally difficult to avoid and this would result in residual haze in the polymer, as TiO2 has a very high refractive index compared to the polymer.
The majority of commercial PET production is still based on Sb-catalysts, and there thus remains a need in industry for a process for making polyesters that uses more environmentally friendly catalysts, with high productivity in melt-phase and/or solid-state polycondensation and low amount of side reactions, which provides polyester articles with good transparency and colour.